1. Field of the Invention
The present invention relates to an optical deflector, an optical scanning apparatus, and an image forming apparatus used for, for example, forming color images.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 2-16521 discloses an example of a related art case of an optical deflector (deflecting apparatus) used in, for example, a color image forming apparatus. The deflecting apparatus includes: plural polygon mirrors for deflecting plural light beams, respectively; a connecting part formed equal to or smaller than an inscribed circle of the plural polygon mirrors for connecting the plural polygon mirrors; and a drive motor that drives rotatively. Thus, the polygon mirrors form a united body having a two level configuration with one polygon mirror on top of the other. However, since the polygon mirrors are fixed to a rotating shaft by a leaf spring member, the temperature rise and centrifugal force generated by high speed rotation of the rotating shaft causes the fixed polygon mirrors to slightly shift and change the balance of the rotated configuration. This results in the problem of vibration. Particularly, a significant vibration may be created even by a slight shift of the polygon mirrors since the mass of the polygon mirrors having the two-level configuration is heavy.
The optical deflector used in a color image forming apparatus includes a type configured to have plural laser beams incident thereto and deflect the plural incident laser beams. Therefore, this type of optical deflector either has the reflection surface of the polygon mirror formed with a large area (increasing thickness of mirror, thick mirror configuration) or has two separate mirrors spaced apart from each other in the axis direction (double mirror configuration). Meanwhile, a polygon scanner having the thick mirror configuration or the double mirror configuration is to be rotated at a high speed of 25,000 rpm or more with high precision for enabling a color image forming apparatus to achieve high speed printing and provide image quality of high definition. However, as the thick mirror configuration is rotated at high speed, windage loss created by the mirror becomes greater. As a result, the windage loss causes the electric power consumption of the motor to increase. Furthermore, with the double mirror configuration, the reference plane during the mounting of the mirrors is required to be processed (finished) with high precision. Furthermore, the optical face tangle between the two mirrors is also required to be set with high precision. Such complicated process/assembly steps lead to a problem of increased manufacturing cost of the double mirror configuration.
Furthermore, as described above, in a case where the configuration having two levels of polygon mirrors forming a united body is rotated at high speed, the temperature rise and centrifugal force generated by the high speed rotation cause the polygon mirrors to slightly shift, change the balance of the rotated configuration, and result in the problem of vibration. This is caused by the polygon mirrors being fixed to a rotating shaft by a leaf spring member. Particularly, a significant vibration may be created even by a slight shift of the polygon mirrors since the mass of the polygon mirrors having the two-level configuration is large. Furthermore, the slight shifting (balance change of the rotated configuration) and the resulting increase of vibration occurring when rotating the configuration at a high speed in a high temperature environment are caused by, for example, the different coefficients of thermal expansion of the components of the rotated configuration (polygon mirror, a flange fixing a rotary magnet, rotating shaft) or (even in a case where the coefficients of thermal expansion of the components match) tolerance and/or the method of the fixing components.
With respect to the above-described problem, Japanese Laid-Open Patent Application No. 2003-177346 (filed by applicant) discloses the below-described optical deflector for providing a polygon scanner and its processing method for achieving high speed rotation with low vibration at high temperature as well as reduction of power consumption and facilitation of assembly.
In the optical deflector disclosed in Japanese Laid-Open Patent Application No. 2003-177346, a rotary member 8 of a polygon scanner 1 includes polygon mirror reflecting surfaces 8a, 8b separated in an axial direction, a circumferential surface fixing a rotary magnet 11 of an outer rotary motor, and a circumferential surface fixing a bearing shaft 10, in which the polygon mirror reflecting surfaces 8a, 8b and the circumferential surfaces are formed of a single member. The rotary member 8 has circumferential grooves 8h, 8i, and 8k which are used as adhesive coating parts for balance correction and prevention of stress strain against the reflecting surfaces 8a, 8b that occur during shrinkage fitting of the bearing shaft 10 or in correspondence with changes of temperature of the environment. Furthermore, the polygon mirror reflecting surfaces 8a, 8b have upper and lower surfaces of a substantially center part shaped substantially as a concave or a convex part with respect to both ends of a part contributing to deflection in a main scanning direction, to thereby prevent deviation of respective colors of a color image forming apparatus.
Although the optical deflector disclosed in Japanese Laid-Open Patent Application No. 2003-177346 is able to reduce windage loss of the mirrors that have their sizes increased along with the increase in high speed rotation by forming a two level configuration by removing an intermediate part of the separated reflecting surfaces, it is difficult to increase the high speed rotation further. In addition, environmental burdens such as power consumption and noise pollution are becoming greater as the high speed rotation is further increased.
Furthermore, although the optical deflector disclosed in Japanese Laid-Open Patent Application No. 2003-177346 is provided with a polygon mirror having 5, 6, or more surfaces for increasing the number of scans per rotation and accelerating printing speed, the reflection surface is to have its width increased to some extent in order to obtain a desired scanning width for the image forming area of the light beam. This results to a problem where the size of the radius of the inscribed circle of the polygon mirrors becomes large. This leads to increase the windage loss of the mirrors.
Furthermore, due to the above-described increase in the radius of the inscribed circle of the polygon mirrors, deformation of the mirrors caused by centrifugal force becomes greater as rotational speed is increased. This results in deterioration of profile regularity. Furthermore, the starting time for the mirrors to reach a predetermined number of rotations (rpm) for scanning becomes longer the more the rotational speed is increased. This increases the amount of power consumed before reaching an actual operating state.